The invention relates to an exhaust-gas cleaning system for cleaning the exhaust gas from a combustion source so as to remove at least nitrogen oxides which are contained therein comprising an ammonia-generating catalytic converter for generating ammonia using constituents of at least some of the exhaust gas emitted from the combustion source during ammonia-generation operating phases, and a nitrogen oxide reduction catalytic converter which is connected downstream of the ammonia-generation catalytic converter, for reducing nitrogen oxides which are contained in the exhaust gas emitted from the combustion source using the ammonia generated as reducing agent therefor.
Exhaust-gas cleaning systems of this type are used in particular for exhaust-gas cleaning in motor vehicle internal-combustion engines and are described, for example, in publications EP 0 802 315 A2 and WO 97/17532 A1. They include a nitrogen oxide reduction catalytic converter for the selective catalytic reduction of nitrogen oxides which are contained in the exhaust gas emitted from the combustion source using ammonia as reducing agent, referred to for short as the SCR process. In order that it is not necessary to hold a stock of ammonia or a precursor in a tank, an ammonia-generation catalytic converter is connected upstream of the nitrogen oxide reduction catalytic converter, the ammonia-generation catalytic converter generating the ammonia which is required using constituents of at least some of the exhaust gas which is emitted from the combustion source during corresponding ammonia-generation operating phases, specifically by means of a synthesis reaction of hydrogen and nitrogen monoxide. In these ammonia-generation operating phases, a rich air ratio is set for the exhaust gas which is fed to the ammonia-generation catalytic converter, in order that sufficient hydrogen be available. In this context, the terms rich and lean air ratio, also known as the lambda value, are understood as meaning, as is customary, a composition of the exhaust gas or of the associated fuel mixture burnt in the combustion source which deviates from the stoichiometric composition by being fuel-rich or oxygen-rich, respectively. In this context, if only for fuel consumption reasons, it is desirable for the combustion source to be operated as much as possible in lean-burn mode and as little as possible in rich-burn mode, for example as a result of prolonged lean-burn operating phases alternating with brief rich-burn operating phases or, in the case of a multicylnder internal-combustion engine, only some of the cylinders, and preferably likewise only from time to time, being operated in rich-burn mode, whereas the other cylinders are continuously operated in lean-burn mode.
The ammonia-generation catalytic converter used is usually a three-way catalytic converter which contains as the catalyst material, by way of example, Pt and/or Rh supported on xcex3-Al2O3, which is suitable for catalysing the synthesis reaction of hydrogen and nitrogen monoxide to form ammonia. One difficulty of this exhaust-gas cleaning technique with internal ammonia generation is that, of the two exhaust-gas constituents which are required for the ammonia synthesis, namely hydrogen and nitrogen monoxide, the hydrogen content increases as the exhaust-gas composition becomes richer, while the nitrogen monoxide content decreases. In order to have sufficient hydrogen available in the exhaust gas for the ammonia synthesis, it is necessary to set a rich exhaust-gas composition, which is usually achieved by setting a corresponding operating state of the combustion source. However, the rich exhaust gas then emitted from the combustion source has only a relatively low nitrogen monoxide concentration, and without further measures this limits the synthesis reaction. On the other hand, it is desirable for it to be possible to generate as much ammonia as possible during these rich-burn operating phases, in order to keep the duration of the rich-burn operating phases as short as possible, for reasons of fuel consumption.
Therefore, the invention is based on the technical problem of providing an exhaust-gas cleaning system of the type described in the introduction with which a high yield of synthesized ammonia for use as a reducing agent for nitrogen oxides can be achieved in corresponding ammonia-generation operating phases with a rich composition of the exhaust gas fed to the ammonia-generation catalytic converter.
The invention solves this problem by providing an exhaust-gas cleaning system wherein a nitrogen monoxide generation unit is situated outside the combustion source and is operable to enrich the exhaust gas fed to the ammonia-generation catalytic converter with nitrogen monoxide during the ammonia-generation operating phases therefor. This system has a nitrogen monoxide generation unit outside the combustion source, which unit, during the ammonia-generation operating phases, enriches the exhaust gas fed to the ammonia-generation catalytic converter with nitrogen monoxide generated by this unit. In this way, in addition to the nitrogen oxide quantity which is already supplied by the combustion process in the combustion source but is limited on account of the rich composition of the fuel mixture burnt, additional nitrogen monoxide generated outside the combustion source is available for the ammonia synthesis in the ammonia-generation catalytic converter. As a result, it is possible to obtain a high yeild of ammonia during the ammonia -generation operating phases with a rich composition of the exhaust gas fed to the ammonia-generation catalytic converter, so that these rich-burn operating phases can be kept relatively short, which keeps fuel consumption at a low level.
The nitrogen monoxide generation unit used may in principle be any desired unit of this type which is familiar to the person skilled in the art. In a refinement of the invention, this unit includes a plasma generator, with which the nitrogen monoxide is generated by oxidation, using plasma technology, of nitrogen contained in a supplied gas stream, which may in particular be an air stream or an exhaust-gas stream formed by at least part of the exhaust gas emitted from the combustion source.